1. Field of the Invention
The present invention generally relates to a sequence block display system used with a sequence controller for displaying sequence blocks on a screen of a cathode-ray tube display unit (herinafter referred to as CRT screen) in the form of ladder diagram.
2. Description of the Prior Art
The prior display system is capable of displaying on a CRT screen desired sequence blocks, each of which is made up of one output element and several input elements, and displaying at the same time the on-off state of the elements. Such a system is disclosed in U.S. Pat. No. 3,964,026. Using the prior display system, an operator can check some of the sequence blocks when a machine trouble occurs. However, it is sometimes difficult to find out the cause of the machine trouble or damaged elements by checking a few sequence blocks which have connection with a certain output element, since an input element which constitutes a sequence block may act as an output element in another sequence block and the on-off state of an input element may depend on the on-off state of other elements. Therefore, it is required to check a lot of sequence blocks one after another which have some connections with the certain output element.
However, the number of sequence blocks displayed is limited due to the size of the CRT screen and accordingly, some sequence blocks previously displayed are put out of the CRT screen when a new sequence block is displayed. Thus the operator, after checking a certain number of sequence blocks, is required to repeat the previous manipulation for display when he desires to check the erased sequence blocks again. Furthermore, the operator must memorize the memory address of the output element of the erased sequence block for subsequently displaying the same on the CRT screen. This requires a large expenditure of time and for that reason is impractical.